Download Anatomy of thyroid gland Surgeon`s perspective

ISSN :2250-0359
Volume 5 Issue 3.5 2015
Anatomy of thyroid gland Surgeon’s perspective
Balasubramanian Thiagarajan
Stanley Medical College
Abstract:
Thyroid surgery can give rise to complications if
performed without adequate knowledge of anatomy. With adequate knowledge of its anatomy,
its variations and optimal operative technique
complications can be reduced. Structures that
should be protected at all costs include the recurrent laryngeal nerves, external branches of
superior laryngeal nerves and parathyroid
glands. Safe surgery involves meticulous hemostasis, and performing the dissection in a meticulous and step wise pattern. Thyroid gland if
pulled medially will help in lateral dissection, securing middle thyroid vein (which may cause
troublesome bleed in some patients as it is a
branch from internal jugular vein).
This also protects the recurrent laryngeal nerve
from damage. Capsular dissection helps in protecting the recurrent laryngeal nerve and also
protects the blood supply of parathyroid glands.
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Introduction:
Thyroid gland was first described by Vesalius in
1543. He called it “Glandulare laryngis” since he
erroneously assumed that this gland lubricated
the larynx. The word thyroid originated from the
Greek word “Thyreos” meaning shield. This
word was first coined by Thomas Wharton in
1656.
History of thyroid surgeries make a good reading.
Thyroid surgeries have been described in the 1st
century AD. Celsus the Roman encyclopedia of
Medicine described the operation as a dangerous procedure. Albucasis 11th century (surgeon
from Corodoba Spain) explained extirpation of
the thyroid gland using setons (threads) passing
through the mass causing it to necrose.
During the 18th century thyroid surgeries were
performed only for life threatening conditions.
Mortality rate those days was more than 20%
which added to its disrepute. In 1850 the French
Academy of Medicine totally banned thyroid surgeries because of its high degree of mortality 1.
The first documented partial thyroidectomy was
performed by Pierre Joseph Dasault in 1791. He
used vertical midline neck incision to remove one
lobe of thyroid gland. He ligated superior and
inferior thyroid arteries and dislodged the gland
from trachea according to his description.
Thyroid surgery became safer due to the advent
of Ether anesthesia in 1846, introduction of antisepsis in 1867, and development of better artery
clamps 1870 which enabled unhurried dissection
of the gland thereby increasing the safety of the
procedure.
Burned seaweed ash was used to treat goiter because of its iodine content in 1811. Modern thyroid surgery was pioneered by Theodar Kocher
who is considered to be a father of thyroid surgery. He reported a series of 146 thyroidectomies spanning between 1850 – 1877 with a mortality rate of 21%. He had impressive statistics.
By the year 1912 he had performed more than
2000 thyroid surgeries 2. He was awarded the
Nobel Prize for his contributions towards the understanding of physiology of thyroid gland.
Towards the beginning of 20th century Thomas
Dunhill Peel described near total thyroidectomy
as a treatment for thyrotoxicosis. He documented about 200 thyroidectomies performed by him
as a treatment for exophthalmic goiter. He even
claimed a mortality rate of less than 3% which
was not believed by surgeons those days.
Developments that pioneered safe thyroid surgery:
Introduction of General anesthesia – by J.C. Warren in 1846 3
Discovery of antisepsis by Lister in 1867 3
Asepsis superseded antisepsis by usage of sterile
instruments and dressings.
.
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Buchner introduced boiling of surgical instruments, and surgeon’s hand washing and cleansing was introduced at Bergman’s clinic by Schimmelbuch.
Anatomy of thyroid gland an overview:
Thyroid gland is a butterfly shaped endocrine
gland situated in the anterior neck. It lies deep
to the platysma, sternothyroid and Sternohyoid
muscles. Commonly this gland extends from the
6th cervical vertebrae to the 1st thoracic vertebrae (C6 – T 1). It has two lobes interconnected
by a thin sleeve of thyroid tissue known as the
isthmus. This thin sleeve of thyroid tissue overlies the 2nd, 3rd and 4th tracheal rings. Isthmus
hence needs to be sectioned or retracted out of
the way when performing tracheostomy, because the stoma will have to be created between
the 3rd and 4th tracheal rings. Commonly the
isthmus measures 2 cm in width, 2 cms in height
and 2-6 mm in thickness 4.
Normal thyroid gland weighs about 15-25 g in
adults. Superior pole of the gland lies lateral to
inferior constrictor muscle and posterior to sternothyroid muscle. The inferior pole of thyroid
gland extends up to the level of 5th or 6th tracheal
rings. Postero laterally the gland overlaps the
carotid sheath and its contents. While
attempting to remove large thyroid masses this
relationship should be borne in mind, to prevent
damage to structures within the carotid sheath.
Image showing the lobes and isthmus of thyroid
gland
Beneath the deep cervical fascia, the thyroid
gland is surrounded by a true inner capsule. This
capsule is thin and is closely adherent to the
gland. Projections rising from the capsule penetrates the thyroid gland dividing it into lobes and
lobules.
A pyramidal lobe may be present in about 50% of
patients. It can arise from either lobe / isthmus
and is directed upwards usually to the left.
Levator glandulae thyroideae is a thin strip of
muscle if present could be seen extending from
the isthmus / lateral lobe to the hyoid bone or
thyroid cartilage. This muscle could be innervated by a branch from ansa cervicalis or superior
laryngeal nerve. Presence of levator glandulae
thyroideae is often associated with the presence
of pyramidal lobe.
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Knowledge about levator glandulae thyroideae is
important during neck surgeries. This structure
could represent the persisting distal end of thyroglossal duct. Anterior cervical region should
hence be clearly examined during total thyroidectomy in order not to leave behind residual thyroid tissue.
Lobes of thyroid gland:
Figure showing thyroid lobe along with pyramidal lobe
Levator glandulae thyroideae is fibrous / fibromuscular band of tissue stretching from the pyramidal lobe / upper border of isthmus of thyroid
gland commonly on the left side. Superiorly it
gets attached to the body of hyoid bone 5.
Mori after performing meticulous dissection of
thyroid tissue classified levator glandulae thyroideae into 5 types according to its attachments.
Hypopyramidalis
Thyreopyramidalis
Thyreoglandularis
Hyoglandularis
Tracheoglandularis
Each lateral lobes of thyroid gland are medially
bound by trachea and oesophagus. Trachea lies
anterior to the oesophagus. Carotid sheath lies
postero lateral to thyroid lobe. Sternomastoid
muscle and other strap muscles (Sternohyoid,
sternothyroid, and superior belly of omohyoid)
lie anterolateral to the thyroid lobe.
Thyroid capsule:
Thyroid gland is ensheathed by true capsule.
This capsule is tightly adherent to the gland, in
fact penetrates the gland. Posteriorly thyroid
gland is anchored by the presence of condensed
middle layer of deep cervical fascia (known as
Berrys ligament / ligament of gruber). This ligament connects the lobes of thyroid gland to cricoid cartilage and first two tracheal rings. Recurrent laryngeal nerve often passes through this
ligament to reach the larynx. This ligament also
contains the terminal branches of inferior thyroid artery. Care must be taken while dissecting
in the area of Berry’s ligament in order to avoid
damage to the recurrent laryngeal nerve 6. It is
advisable to dissect this area under magnification
to prevent damage to recurrent laryngeal nerve.
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Studies reveal that this ligament may contain residual thyroid tissue and hence needs to be removed in total thyroidectomy.
Blood supply:
Blood supply to thyroid gland is derived from a
pair of arteries: i.e. Superior and Inferior thyroid
arteries. Superior thyroid artery the first branch
of external carotid artery along with superior
thyroid vein supplies the upper pole of the thyroid gland. Close to the upper pole this artery
lies superficial to the external laryngeal branch of
superior laryngeal nerve which courses to supply
the cricothyroid muscle. Rarely in 10% of cases
superior thyroid artery may arise directly from
common carotid artery 7. Superior thyroid artery
passes deep to infrahyoid muscles, and on reaching the superior pole of thyroid gland, divides
into anterior and posterior branches to supply
superior portion of thyroid gland. Before these
branches are given off superior thyroid artery
gives off superior laryngeal artery which passes
across the thyrohyoid membrane along with superior laryngeal nerve enters the larynx. It also
gives off another branch i.e. cricothyroid artery
near the lower border of thyroid cartilage. This
artery lies on the cricothyroid membrane.
Inferior thyroid artery is larger than that of superior thyroid artery. It is a branch of thyrocervical
trunk, which is a branch of subclavian artery.
This artery courses along the anterior scalene
muscle, turning medially behind the common
carotid artery descending on the posterior aspect of lateral lobes of thyroid before entering
the inferior thyroid pole.
In its course behind the common carotid artery
this vessel exhibits a variable relationship with
that of the sympathetic chain. This vessel may
be absent in 5% of patients.
Another inconsistent artery is arteria thyroideae
ima. This vessel could replace inferior thyroid
artery. This vessel could arise from:
Innominate artery
Subclavian artery
Right common carotid artery
Internal mammary artery
Aortic arch
This artery is more common in the right side.
Since it is closely related to the anterior wall of
trachea it may cause troublesome bleeding during tracheostomy.
Within the capsule of thyroid gland there is a
dense network of interconnecting blood vessels.
These vessels form extensive capillary plexus
around the follicles. Veins draining these plexuses give rise to inferior, middle and superior thyroid veins. These veins in turn join either the internal jugular or innominate vein. It is precisely
these veins that cause bleeding during thyroidectomy or tracheostomy procedures.
Lymphatics draining the thyroid gland usually
follow the veins. Those travelling along with superior and middle thyroid veins drain into upper
and middle deep cervical nodes respectively.
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Lymphatics following inferior thyroid vessels
drain into lower deep cervical chain of nodes,
supraclavicular, pretracheal and prelaryngeal
nodes.
Thyroid gland is purely innervated by sympathetic nervous system from superior, middle and
inferior cervical ganglia.
Parathyroid glands:
Parathyroid glands which secrete parathormone
are typically 4 in number i.e. two superior and
two inferior. 10% of patients may have supernumerary parathyroid glands. Each parathyroid
gland weighs 35-40 mg measuring about 3-8 mm
in all three dimensions. These glands can vary in
color between yellow to reddish brown. Inferior
parathyroid gland is found within 1 cm inferior,
lateral or posterior to the inferior pole of thyroid
gland in at least 50% of cases.
Usually inferior parathyroid gland lies anterior to
a plane drawn along the course of recurrent laryngeal nerve 8. Commonly inferior parathyroid
glands could also be seen within the capsule of
thyroid gland. Superior parathyroid gland can be
found in the posterior aspect of thyroid lobe just
1 cm above the crossing of inferior thyroid artery
and recurrent laryngeal nerve. Superior parathyroid gland could be commonly seen located in
the posterolateral aspect of superior pole of thyroid gland at the cricothryoid cartilage junction.
Vascular anatomy of parathyroid glands:
Parathyroid glands are highly vascular structures.
The inferior parathyroid gland is supplied by inferior thyroid artery. In 10% of patients’ inferior
thyroid artery could be absent (particularly on
the left side).
In such cases a branch from superior thyroid artery supplies inferior parathyroid glands also.
Whereas superior parathyroid glands are supplied by superior thyroid artery. The location of
inferior parathyroid shows the greatest degree of
variation because inferior parathyroid glands migrate with the thymus which descends from the
angle of mandible to the pericardium.
Damage to recurrent laryngeal / superior laryngeal nerves during thyroid surgeries leave behind
a troublesome morbidity. Care should be taken
to ensure that these two important structures
are not damaged during surgery.
Superior laryngeal nerve:
This nerve originates at the inferior ganglion of
vagus nerve (nodose ganglion) close to the jugular foramen. The average length of superior laryngeal nerve is about 2 cm in males and 1.5 cms
in females. It courses posterior and medial to
the internal carotid artery and descends antero
inferiorly to reach the larynx. At the level of
greater cornu of hyoid bone it divides into a large
internal laryngeal and a smaller external laryngeal branch. The internal laryngeal nerve passes
between thyrohyoid muscle and the thyrohyoid
membrane. It pierces the thyrohyoid membrane
along with superior laryngeal artery and vein to
supply sensation to the interior of larynx.
It supplies sensation to supraglottis and pyriform
sinus. After entering into the larynx this nerve
divides into three branches i.e. superior, middle
and inferior.
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The superior division divides into two / three
branches supplying sensations to the lingual surface of epiglottis, lateral aspect of glosso epiglottic fold. The middle division innervates the
aryepiglottic fold, vocal folds, vestibular folds
and the posterior aspect of arytenoid. The inferior division is the largest of the branches of superior laryngeal nerve. It lies along the medial aspect of pyriform fossa. It is this nerve which is
blocked when pyriform fossa block is given for
endolaryngeal surgical procedures. This branch
supplies the interarytenoid muscle. This nerve
gives out a branch that communicates with the
recurrent laryngeal nerve (Galen's loop). The
external laryngeal nerve at the level of superior
horn of thyroid cartilage turns medially and runs
posterior and parallel to the oblique line. This
nerve lies deep to the superior thyroid artery. Its
relationship with the superior pole of thyroid
gland is highly variable.
Type III nerve: In this type the external branch of
superior laryngeal nerve crosses the superior
thyroid artery under cover of the superior pole
of thyroid gland.
Kierner classified the superior laryngeal nerve
into 4 types depending on the relationship of its
external branch to the superior pole of thyroid
gland.
The following are the steps which should be taken during thyroid surgeries to minimize damage
to external laryngeal nerve:
Type I nerve: In this type the external branch of
superior laryngeal nerve cross the superior thyroid artery about 1cm above the superior pole of
thyroid gland.
Type IV nerve: In this type the external branch of
superior laryngeal nerve descends dorsal to the
superior thyroid artery and crosses its branches
just superior to the upper pole of thyroid gland.
Awareness of these anatomical variations will
help the surgeon in preserving this branch during
head and neck surgeries.
Incidence of superior laryngeal nerve injury during thyroid surgeries run close to 40% 9. The incidence is also probably underdiagnosed and under reported. Prevention of damage to external
laryngeal nerve has not be emphasized.
1. Division of sternothyroid muscle
2. Careful dissection in the cricothyroid space
(space that lies medial surface of thyroid
gland and cricothyroid muscle.
3. Meticulous isolation and dissection of superior thyroid vessels
Type II nerve: In this type the external branch of
superior laryngeal nerve crosses the superior
thyroid artery within 1 cm of the superior pole of
thyroid gland.
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Recurrent laryngeal nerves:
Conclusion:
Always asymmetric. Left recurrent laryngeal
nerve arises from vagus in the chest. It crosses
the arch of aorta to reach the neck. It ascends
posterior to the thyroid gland to reach the tracheo oesophageal groove. On the right side recurrent laryngeal nerve arises from vagus in the
chest and reaches the neck by crossing anterior
to right subclavian artery. It loops around the
right subclavian artery to reach the tracheo oesophageal groove posterior to thyroid gland. It
enters the larynx behind the cricothyroid articulation and inferior cornua of thyroid. Left recurrent laryngeal nerve is more closely related to
the trachea in its lower part when compared to
that of the right nerve. At the lower pole of thyroid gland right nerve is slightly more anterior
than the left. During the middle part of the
course recurrent laryngeal nerve is found within
the tracheo oesophageal groove commonly. Intraoperatively recurrent laryngeal nerves could
be seen in the Simon triangle formed by the
common carotid artery laterally, oesophagus medially and inferior thyroid artery superiorly. It
can also be identified with certainty where it enters the larynx just behind the inferior cornua of
thyroid cartilage.
In order to prevent complications and unnecessary post-operative morbidity following thyroid
surgery one must have through understanding of
anatomy of thyroid glands and their adnexal
structures. Surgeon should learn to avoid pitfalls
by following a systematic approach to identify
vulnerable structures.
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4. Hoyes AD, Kershaw DR. Anatomy and development of the thyroid gland. Ear
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drtbalusotolaryngology/Home/
laryngology/surgical-anatomy-of-thyroid
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Eur Arch Otorhinolaryngol 2001;258:451–4.
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